In a time of flight mass spectrometer (TOF-MS), ions accelerated by an electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass-to-charge ratios according to the flight time until they reach and are detected by a detector. Since the difference of the lengths of flight time of two ions having different mass-to-charge ratios is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the resolution of the mass-to-charge ratio of a TOF-MS. In many cases, however, it is difficult to incorporate a long straight path in a TOF-MS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length.
In the Japanese Unexamined Patent Publication No. H11-297267, an elliptical orbit is formed using plural toroidal type sector-formed electric fields, and the ions are guided to fly repeatedly in the elliptical orbit many times, whereby the effective flight length is elongated. In the Japanese Unexamined Patent Publication No. H11-135061, ions fly in an approximately “8” shaped orbit repeatedly. In these TOF-MSs, as the number of turns the ions fly in the orbit increases, the flight distance is larger and the length of flight time is accordingly longer, so that the resolution of the mass-to-charge ratio becomes better by increasing the number of turns.
When, as described above, ions repeatedly fly in a loop orbit, ions having smaller mass-to-charge ratios will gain higher speeds. Therefore, ions having a smaller mass-to-charge ratio may lap other ions having larger mass-to-charge ratios while they are orbiting. If the detector simultaneously detects a group of ions mixed with different number of turns, it is impossible to determine the mass-to-charge ratios of the ions without knowing the number of turns of each ion. One conventional solution to such a problem is to limit the range of the mass-to-charge ratio of the ions brought into the loop orbit in order to avoid ions having such a diversity of mass-to-charge ratios that causes the lapping problem. In this method, if the analysis should cover a broad range of mass-to-charge ratios, it is necessary to divide the range of the mass-to-charge ratio into smaller segments and carry out the analysis many times. If there is only a limited amount of sample available for the analysis, it is difficult to carry out the analysis many times, meaning that the analysis cannot be carried out over the broad range of the mass-to-charge ratio.
To limit the mass-to-charge ratio of the ions before they are brought into the loop orbit, it is necessary to roughly separate the ions by their mass-to-charge ratios before they enter the loop orbit. One possible method is to use an ion trap or other device capable of separating the ions. However, mass spectrometers are not always constructed to allow the use of an ion trap or other ion separator. Another possible method is to make the distance between the ion source and the loop orbit large enough to allow the limitation of the range of mass-to-charge ratios before the ions enter the loop orbit. However, it is not always allowable to keep a large distance between the ion source and the loop orbit because the overall size of the mass spectrometer is limited.